Project Summary A ?connectome? describes the complete synaptic wiring diagram of a brain. The elucidation of the connectome of any animal brain and, ultimately, the human brain will have a tremendous impact on our understanding of brain function and constitutes a central goal of 21st century neuroscience, akin to the efforts to assemble the complete sequence of genomes. Current connectomic efforts are focused on determining the anatomical synaptic connections between neurons in a brain, thereby completely ignoring aspects of neuronal communication that are likely of equal importance but are not captured by anatomical connections: Neuromodulatory communication by neuropeptides and their cognate receptors. Neuropeptidergic communication is usually non-synaptic, i.e. neuropeptides are often released from non-synaptic sites and cognate neuropeptide receptors are often located distal from the source of the cognate neuropeptide. While the importance of a number of neuropeptides and their receptors in controlling behavior are well appreciated, the extent of usage of neuropeptidergic signaling is only beginning to be fully appreciated. Every neuron in an animal nervous system is now thought to express at least one neuropeptide, but the pathways of communication of these neuropeptidergic signals have not been comprehensibly mapped and, hence, our understanding of information flow in the nervous system remains limited. We propose here to use C. elegans as a model system to establish the first comprehensive neuropeptidergic connectome. The simplicity of the C. elegans nervous system allows to undertake such a comprehensive analysis and, importantly, allows to compare a neuropeptidergic connectome to that of the completely established synaptic connectome. Based on preliminary data we expect to describe a ?multilayer connectome? with substantially distinct pathways of information flow, as well as distinct and similar topological features. We will achieve to build such a connectome through (1) comprehensively defining ligand/receptor pairs through in vitro receptor activation assays, (2) defining the expression patterns of all neuropeptide and neuropeptide receptor encoding genes, (3) synthesizing these data into a neuropeptidergic network and computationally comparing the topology of this network to the synaptic connectivity network and (4) undertaking a preliminary functional validation of specific nodes and edges of this network.